Photoresist and method

ABSTRACT

Multi-layer photoresists, methods of forming the same, and methods of patterning a target layer using the same are disclosed. In an embodiment, a method includes depositing a reflective film stack over a target layer, the reflective film stack including alternating layers of a first material and a second material, the first material having a higher refractive index than the second material; depositing a photosensitive layer over the reflective film stack; patterning the photosensitive layer to form a first opening exposing the reflective film stack, patterning the photosensitive layer including exposing the photosensitive layer to a patterned energy source, the reflective film stack reflecting at least a portion of the patterned energy source to a backside of the photosensitive layer; patterning the reflective film stack through the first opening to form a second opening exposing the target layer; and patterning the target layer through the second opening.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of U.S. Provisional Application No.62/968,483, filed on Jan. 31, 2020, which application is herebyincorporated herein by reference.

BACKGROUND

Semiconductor devices are used in a variety of electronic applications,such as, for example, personal computers, cell phones, digital cameras,and other electronic equipment. Semiconductor devices are typicallyfabricated by sequentially depositing insulating or dielectric layers,conductive layers, and semiconductor layers of material over asemiconductor substrate, and patterning the various material layersusing lithography to form circuit components and elements thereon.

The semiconductor industry continues to improve the integration densityof various electronic components (e.g., transistors, diodes, resistors,capacitors, etc.) by continual reductions in minimum feature size, whichallow more components to be integrated into a given area.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 5 illustrate cross-sectional views of intermediarystages of manufacturing a semiconductor device in accordance with someembodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Various embodiments provide improved multi-layer photoresists, methodsof forming the same, and methods of patterning target layers using themulti-layer photoresists. The multi-layer photoresists may include areflective film stack and a photosensitive layer over the reflectivefilm stack. The reflective film stack may be a multi-layer film stackwhich includes alternating first material films and second materialfilms, which are formed of different materials. In some embodiments, thefirst material films may include high refractive index materials and thesecond material films may include low refractive index materials. Forexample, the first material films may include silicon (Si), beryllium(Be), or the like and the second material films may include molybdenum(Mo) or the like. Forming the reflective film stack under thephotosensitive layer may allow for the photosensitive layer to beexposed using a lower energy dose with less exposure time. This reducesthe energy and time required to expose the photosensitive layer,increasing throughput and reducing manufacturing costs and improvesline-width and line-edge integrity, reducing device defects andincreasing device performance.

FIGS. 1 through 5 illustrate cross-sectional views of intermediatestages in the formation of features in a target layer 102 in asemiconductor device 101, in accordance with some embodiments. In someembodiments, the semiconductor device 101 may be processed as part of alarger wafer. In such embodiments, after various features of thesemiconductor device 101 are formed (e.g., active devices, interconnectstructures, and the like), a singulation process may be applied toscribe line regions of the wafer in order to separate individualsemiconductor dies from the wafer (also referred to as singulation).

As illustrated in FIG. 1, the target layer 102 may be formed over asemiconductor substrate 100. The semiconductor substrate 100 may beformed of a semiconductor material such as silicon, doped or undoped, oran active layer of a semiconductor-on-insulator (SOI) substrate. Thesemiconductor substrate 100 may include other semiconductor materials,such as germanium; a compound semiconductor including silicon carbide,gallium arsenic, gallium phosphide, indium phosphide, indium arsenide,and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP,AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; combinations thereof; orthe like. Other substrates, such as multi-layered or gradientsubstrates, may also be used. Devices, such as transistors, diodes,capacitors, resistors, and the like, may be formed in and/or on anactive surface of the semiconductor substrate 100. In other embodiments,where the target layer 102 is a semiconductor substrate used to form finfield-effect transistors (FinFETs), the semiconductor substrate 100 maybe omitted.

The target layer 102 may be a layer in which a pattern is to be formed.In some embodiments, the target layer 102 may be a conductive layer, adielectric layer, a semiconductor layer, or the like. In embodiments inwhich the target layer 102 is a conductive layer, the target layer maybe a metal layer, a polysilicon layer, or the like. The target layer 102may be deposited by physical vapor deposition (PVD), chemical vapordeposition (CVD) (e.g., blanket deposition or the like), or the like.The conductive layer may be patterned according to the processesdescribed below to form metal gates (e.g., in a cut metal gate process),conductive lines, conductive vias, dummy gates (e.g. for replacementgates in FinFETs), or the like.

In embodiments in which the target layer 102 is a dielectric layer, thetarget layer 102 may be an inter-metal dielectric layer, an inter-layerdielectric layer, a passivation layer, or the like. The target layer 102may be a material having a low dielectric constant (e.g., a low-kmaterial). For example, the target layer 102 may have a dielectricconstant lower than 3.8, lower than about 3.0, or lower than about 2.5.The target layer 102 may be a material having a high dielectricconstant, such as a dielectric constant higher than 3.8. The targetlayer 102 may be deposited by chemical vapor deposition (CVD), atomiclayer deposition (ALD), or the like. One or more openings (such asopenings 110, discussed below with respect to FIG. 5) may be patternedin the target layer 102 according to the processes described below andconductive lines, conductive vias, or the like may be formed in theopenings in the target layer 102.

In embodiments in which the target layer 102 is a semiconductormaterial, the target layer 102 may be formed of silicon, silicongermanium, or the like. In some embodiments, the target layer 102 may beformed of a crystalline semiconductor material such as crystallinesilicon, crystalline silicon carbide, crystalline silicon germanium, acrystalline III-V compound, or the like. In some embodiments, openings(such as openings 110, discussed below with respect to FIG. 5) may bepatterned in the target layer 102 according to the processes describedbelow and shallow trench isolation (STI) regions may be formed in theopenings in the target layer 102. Semiconductor fins may protrude frombetween neighboring STI regions and source/drain regions may be formedin the semiconductor fins. The semiconductor fins may include materialof the target layer 102 remaining after forming the openings in thetarget layer 102. Gate dielectric layers and gate electrodes may beformed over channel regions in the semiconductor fins, thereby formingsemiconductor devices such as fin field effect transistors (FinFETs).

Although FIG. 1 illustrates the target layer 102 as being in physicalcontact with the semiconductor substrate 100, any number of interveninglayers may be disposed between the target layer 102 and thesemiconductor substrate 100. Such intervening layers may include aninter-layer dielectric (ILD) layer, which may include a low-k dielectricand may include contact plugs formed therein, other inter-metallicdielectric (IMD) layers having conductive lines and/or vias formedtherein, one or more intermediary layers (e.g., etch stop layers,adhesion layers, or the like), combinations thereof, and the like. Forexample, an optional etch stop layer may be disposed directly under thetarget layer 102. The etch stop layer may act as a stop for an etchingprocess subsequently performed on the target layer 102 (e.g., theetching process described below with respect to FIG. 4). The materialsand processes used to form the etch stop layer may depend on thematerial of the target layer 102. In some embodiments, the etch stoplayer may be formed of silicon nitride, SiON, SiCON, SiC, SiOC,SiC_(x)N_(y), SiO_(x), other dielectrics, combinations thereof, or thelike, and may be formed by chemical vapor deposition (CVD), atomic layerdeposition (ALD), or the like.

A reflective film stack 104 is formed over the target layer 102. Thereflective film stack 104 is a multi-layer film stack which includesalternating layers of high refractive index films 104A and lowrefractive index films 104B. The high refractive index films 104A andthe low refractive index films 104B may be deposited by physical vapordeposition (PVD), chemical vapor deposition (CVD), atomic layerdeposition (ALD), or the like. The high refractive index films 104A maybe formed of high refractive index materials and the low refractiveindex films 104B may be formed of low refractive index materials. Thehigh refractive index materials may have a refractive index ranging fromabout 0.94 to about 1.03 or from about 0.97 to about 1.00. The lowrefractive index materials may have a refractive index ranging fromabout 0.87 to about 1.00 or from about 0.90 to about 0.97. A ratio ofthe refractive index of the high refractive index materials to therefractive index of the low refractive index materials may be from about1.00 to about 1.15 or from about 1.05 to about 1.10. The high refractiveindex materials may have a tendency to scatter incoming radiation (suchas the radiation beam 108, discussed below with respect to FIG. 2), andthe low refractive index materials may have a tendency to transmitincoming radiation. Including alternating layers of the high refractiveindex films 104A and the low refractive index films 104B providesreflectivity for the reflective film stack 104. Moreover, includingrefractive index materials having the prescribed refractive indexesimproves the reflectivity of the reflective film stack 104 to incomingradiation in embodiments in which the incoming radiation includesextreme ultraviolet (EUV) radiation.

Various qualities of the reflective film stack 104 may be selected toincrease the reflectivity of the reflective film stack 104 based on thewavelength of incoming radiation. For example, the materials and layerthicknesses of the high refractive index films 104A and the lowrefractive index films 104B, the number of pairs of the high refractiveindex films 104A and the low refractive index films 104B included in thereflective film stack 104, and the overall thickness of the reflectivefilm stack 104 may be selected to increase the reflectivity of thereflective film stack 104, while keeping the thickness of the reflectivefilm stack 104 to a minimum.

In embodiments in which the incoming radiation includes EUV radiation,the high refractive index materials may include silicon (Si), beryllium(Be), or the like. The low refractive index materials may includemolybdenum (Mo) or the like. Each of the high refractive index films104A may have a thickness from about 1 nm to about 6 nm or from about 3nm to about 4 nm. Each of the low refractive index films 104B may have athickness from about 1 nm to about 6 nm or from about 3 nm to about 4nm. A ratio of thicknesses of the high refractive index films 104A tothicknesses of the low refractive index films 104B may be from about 1:2to about 2:1 or from about 1:1 to about 3:2. The reflective film stack104 may include from about 20 to about 80 pairs or from 35 to 45 pairsof the high refractive index films 104A and the low refractive indexfilms 104B. A total thickness of the reflective film stack 104 may befrom about 200 nm to about 450 nm or from about 280 nm to about 360 nm.Although the bottom layer of the reflective film stack 104 isillustrated as being a high refractive index film 104A and the top layerof the reflective film stack 104 is illustrated as being a lowrefractive index film 104B, both the top layer and the bottom layer ofthe reflective film stack 104 may be a high refractive index film 104Aor a low refractive index film 104B.

The materials of the high refractive index films 104A and the lowrefractive index films 104B may also have low extinction coefficientssuch that absorption of incoming radiation by the reflective film stack104 is minimized. For example, extinction coefficients of the highrefractive index films 104A and the low refractive index films 104B maybe less than about 0.10 or less than about 0.01.

A photosensitive layer 106 is deposited over the reflective film stack104. The photosensitive layer 106 may be a photosensitive material andmay be formed of an organic material, such as a polymer material. Thephotosensitive layer 106 may be deposited by spin-on processes, CVD,ALD, or the like. The photosensitive layer 106 may have a thickness fromabout 10 nm to about 80 nm or from about 20 nm to about 60 nm. Thephotosensitive material may be a positive photosensitive material (e.g.,a material in which portions exposed to energy are removed by adeveloper) or a negative photosensitive material (e.g., a material inwhich portions that are not exposed to energy are removed by adeveloper).

In some embodiments, adhesion between the photosensitive layer 106 andthe reflective film stack 104 may be improved by exposing the reflectivefilm stack to an adhesion promoter such as hexamethyldisilazane (HMDS,[(CH₃)₃Si]₂NH) or the like. For example, in some embodiments, thereflective film stack 104 may be exposed to gaseous HMDS, which maycause the surface of the reflective film stack 104 to become silylated.This results in the surface of the reflective film stack being morehydrophobic, which improves adhesion of the photosensitive layer 106 tothe reflective film stack 104.

In some embodiments, a planarity layer may be disposed between thetarget layer 102 and the reflective film stack 104. The planarity layermay be used to planarize the target layer 102, as top surfaces of thetarget layer 102 may be uneven. The planarity layer may be deposited byspin-on processes or the like. In some embodiments, the planarity layermay be deposited by a conformal process, such as spin-coating, chemicalvapor deposition (CVD), atomic layer deposition (ALD), or the like andthe planarity layer may be subsequently planarized using a process suchas chemical mechanical planarization (CMP). The planarity layer may havea thickness from about 50 nm to about 200 nm or from about 100 nm toabout 150 nm. The planarity layer may be formed of a material includingcombinations of silicon, carbon, nitrogen, oxygen, hydrogen, multiplelayers thereof, or the like. For example, the planarity layer mayinclude silicon nitride (SiN), silicon oxide (SiO₂), silicon oxynitride(SiO_(x)N_(y)), or the like.

In some embodiments, a selectivity layer may be disposed between thetarget layer 102 and the reflective film stack 104. The selectivitylayer may be provided to improve etch selectivity between the reflectivefilm stack 104 and the target layer 102. The selectivity layer may bedeposited by chemical vapor deposition (CVD), atomic layer deposition(ALD), or the like. The selectivity layer may have a thickness fromabout 5 nm to about 60 nm or from about 30 nm to about 40 nm. Theselectivity layer may be formed of a material including aluminum,tungsten, hafnium, zirconium, silicon, carbon, nitrogen, oxygen,hydrogen, combinations or multiple layers thereof, or the like. Forexample, the selectivity layer may include TiN, Al₂O₃, or the like. Afirst etch selectivity between the selectivity layer and the targetlayer 102 may be greater than a second etch selectivity between thereflective film stack 104 and the target layer 102. For example, a ratioof the first etch selectivity to the second etch selectivity may begreater than about 2.

In FIGS. 2 and 3, the photosensitive layer 106 is patterned. Thephotosensitive layer 106 may be patterned using lithography techniquessuch as extreme ultraviolet (EUV) lithography, deep ultraviolet (DUV)lithography, X-ray lithography, soft X-ray (SX) lithography, ion beamprojection lithography, electron-beam projection lithography, or thelike. In FIG. 2, the photosensitive layer 106 is exposed to a radiationbeam 108. In some embodiments, the radiation beam 108 may include EUVradiation. For example, the radiation beam 108 may include EUV radiationhaving wavelengths ranging from 10 nm to 125 nm. In some embodiments,the radiation beam 108 may include radiation having a wavelength ofabout 13.5 nm. The radiation beam 108 may be generated from a tin (Sn)plasma which radiates light of the proper wavelength and which may belaser-produced. The radiation beam 108 may be patterned prior toirradiating the photosensitive layer 106. For example, a mask having apattern to be patterned in the target layer 102 or the inverse of thepattern may be used to pattern the radiation beam 108. The mask may be atransmissive mask, a reflective mask, or the like.

Energy from the radiation beam 108 (e.g., EUV photons) that is notabsorbed by a first pass through the photosensitive layer 106 may bereflected onto a backside of the photosensitive layer 106 by thereflective film stack 104. As illustrated in FIG. 2, the radiation beam108 may pass through one or more of the high refractive index films 104Aand/or the low refractive index films 104B of the reflective film stack104 and may be reflected by interfaces between adjacent high refractiveindex films 104A and low refractive index films 104B. The radiation beam108 may be supplied with an energy dose of less than about 30 mJ/cm²,from about 20 mJ/cm² to about 50 mJ/cm², or from about 30 mJ/cm² toabout 40 mJ/cm². The photosensitive layer 106 may be exposed to theradiation beam for a time ranging from about 10 milliseconds to about100 milliseconds or less than about 100 milliseconds.

Reflecting the radiation beam 108 onto the backside of thephotosensitive layer 106 allows for the photosensitive layer 106 to beexposed using a radiation beam 108 having a lower dose in less time ascompared to conventional methods of exposing a photosensitive layer 106.This increases throughput and reduces costs. Moreover, exposing thephotosensitive layer 106 to a lower radiation dose improves line-widthroughness (LWR) and line-edge roughness (LER), which improves deviceperformance and reduces device defects.

In FIG. 3, the photosensitive layer 106 is patterned to form one or moreopenings 110. The photosensitive layer may be patterned by exposing thephotosensitive layer 106 to a developer. The photosensitive layer 106may be a positive tone resist or a negative tone resist. In embodimentsin which the photosensitive layer 106 is a positive tone resist,portions of the photosensitive layer 106 that were exposed to theradiation beam 108 may be removed by exposing the photosensitive layer106 to the developer. In embodiments in which the photosensitive layer106 is a negative tone resist, portions of the photosensitive layer 106that were not exposed to the radiation beam 108 may be removed byexposing the photosensitive layer 106 to the developer. The openings 110may have widths from about 10 nm to about 50 nm or from about 30 nm toabout 40 nm and aspect ratios (e.g., a height-to-width ratio) from about1 to about 6.

In FIG. 4, the reflective film stack 104 and the target layer 102 areetched to extend the openings 110. For example, once the photosensitivelayer 106 has been patterned into the desired pattern, thephotosensitive layer 106 may be used as a mask to pattern the reflectivefilm stack 104. The pattern of the photosensitive layer 106 may betransferred to the reflective film stack 104 using an anisotropic etchprocess such as reactive ion etching (RIE), neutral beam etching (NBE),or the like.

Once the pattern of the photosensitive layer 106 has been transferred tothe reflective film stack 104, the reflective film stack 104 may be usedto transfer the pattern of the photosensitive layer 106 to the targetlayer 102 to form or extend the openings 110 such that the openings 110expose top surfaces of the semiconductor substrate 100. In someembodiments, the target layer 102 may be etched using an etching processthat utilizes both the photosensitive layer 106 and the reflective filmstack 104 (now patterned) as masking layers. The pattern of thephotosensitive layer 106 and/or the reflective film stack 104 may betransferred to the target layer 102 using an anisotropic etch processsuch as reactive ion etching (RIE), neutral beam etching (NBE), or thelike. In some embodiments, the pattern of the photosensitive layer 106may be transferred to the reflective film stack 104 and the target layer102 simultaneously using a single etch process.

In FIG. 5, once the pattern of the photosensitive layer 106 has beentransferred to the reflective film stack 104 and the target layer 102,the photosensitive layer 106 and the reflective film stack 104 areremoved. In some embodiments, an ashing process may be used to removethe photosensitive layer 106 from the reflective film stack 104. In suchembodiments, the temperature of the photosensitive layer 106 may beincreased to cause a thermal breakdown of the photosensitive layer 106,which can then be removed using a cleaning procedure such as a rinse. Insome embodiments, the photosensitive layer 106 may be removed using awet etching process or the like. Any suitable method for removing thephotosensitive layer 106 may be used.

The reflective film stack 104 may be removed using an etching process,such as a wet etching process, a dry etching process, or the like. Inembodiments in which the reflective film stack 104 is removed by a dryetching process, the dry etching process may use gases including carbontetrafluoride (CF₄), sulfur hexafluoride (SF₆), combinations thereof, orthe like.

Forming the reflective film stack 104 under the photosensitive layer 106allows for the photosensitive layer 106 to be exposed to more energy(e.g., EUV photons) from the radiation beam 108, reducing the total doseof the energy required to expose the photosensitive layer 106 andallowing the photosensitive layer 106 to be exposed in less time. Thisresults in improved line-width roughness (LWR) and improved line-edgeroughness (LER), reduces device defects, and improves deviceperformance. Using a lower dose of energy and reducing exposure timealso reduces costs and increases throughput.

In accordance with an embodiment, a method includes depositing areflective film stack over a target layer, the reflective film stackincluding alternating layers of a first material and a second material,the first material having a higher refractive index than the secondmaterial; depositing a photosensitive layer over the reflective filmstack; patterning the photosensitive layer to form a first openingexposing the reflective film stack, patterning the photosensitive layerincluding exposing the photosensitive layer to a patterned energysource, the reflective film stack reflecting at least a portion of thepatterned energy source to a backside of the photosensitive layer;patterning the reflective film stack through the first opening to form asecond opening exposing the target layer; and patterning the targetlayer through the second opening. In an embodiment, a ratio of arefractive index of the first material to a refractive index of thesecond material is from 1.05 to 1.10. In an embodiment, a ratio of athickness of a layer of the first material to a thickness of a layer ofthe second material is from 1:1 to 3:2. In an embodiment, the reflectivefilm stack includes from 35 to 45 pairs of layers of the first materialand the second material. In an embodiment, the patterned energy sourceincludes extreme ultraviolet (EUV) radiation. In an embodiment, a doseof the patterned energy source is less than 30 mJ/cm². In an embodiment,the photosensitive layer is exposed to the patterned energy source forless than 100 milliseconds.

In accordance with another embodiment, a method includes forming amulti-layer photoresist, forming the multi-layer photoresist includingforming a reflective film stack including alternating layers of a firstreflective material and a second reflective material, a ratio of arefractive index of the first reflective material to a refractive indexof the second reflective material being from 1.05 to 1.10; and forming aphotosensitive layer over the alternating layers of the reflective filmstack; patterning the photosensitive layer and the reflective filmstack; and patterning a target layer using the reflective film stack asa mask. In an embodiment, the second reflective material includesmolybdenum (Mo). In an embodiment, the first reflective materialincludes silicon (Si). In an embodiment, the first reflective materialincludes beryllium (Be). In an embodiment, a ratio of thicknesses of thelayers of the first reflective material to thicknesses of the layers ofthe second reflective material is from 1:1 to 3:2. In an embodiment,patterning the photosensitive layer includes exposing the photosensitivelayer to a radiation beam including patterned extreme ultravioletradiation. In an embodiment, the reflective film stack includes from 35to 45 pairs of the first reflective material and the second reflectivematerial. In an embodiment, the radiation beam is reflected at aninterface between the first reflective material and the secondreflective material.

In accordance with yet another embodiment, a method includes forming areflective film stack including alternating layers of a first materialand a second material different from the first material; exposing afront side of a photosensitive layer to a radiation beam, the radiationbeam including extreme ultraviolet (EUV) radiation, a portion of theradiation beam being reflected from an interface between the firstmaterial and the second material of the reflective film stack to abackside of the photosensitive layer; exposing the photosensitive layerto a developer to remove a portion of the photosensitive layer exposingthe reflective film stack; etching the reflective film stack using thephotosensitive layer as a mask; and etching a target layer underlyingthe reflective film stack using the reflective film stack as a mask, thetarget layer including a semiconductor material, a conductive layer, ora dielectric layer. In an embodiment, the method further includesdepositing a planarity layer over the target layer; and planarizing theplanarity layer using a chemical mechanical planarization (CMP) process,the reflective film stack being formed over the planarity layer, and thetarget layer including a non-planar top surface. In an embodiment, themethod further includes exposing the reflective film stack to anadhesion promoter, the adhesion promoter including hexamethyldisilazane(HMDS). In an embodiment, the reflective film stack is formed directlyon the target layer. In an embodiment, the method further includesforming an etch selectivity layer over the target layer, the reflectivefilm stack being formed over the etch selectivity layer, an etchselectivity between the etch selectivity layer and the target layerbeing greater than an etch selectivity between the reflective film stackand the target layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: depositing a reflective filmstack over a target layer, the reflective film stack comprisingalternating layers of a first material and a second material, the firstmaterial having a higher refractive index than the second material;depositing a photosensitive layer over the reflective film stack,wherein the photosensitive layer is deposited in direct contact with thefirst material or the second material; patterning the photosensitivelayer to form a first opening exposing the reflective film stack,wherein patterning the photosensitive layer comprises exposing thephotosensitive layer to a patterned energy source, wherein thereflective film stack reflects at least a portion of the patternedenergy source to a backside of the photosensitive layer; patterning thereflective film stack through the first opening to form a second openingexposing the target layer; and patterning the target layer through thesecond opening.
 2. The method of claim 1, wherein a ratio of arefractive index of the first material to a refractive index of thesecond material is from 1.05 to 1.10.
 3. The method of claim 1, whereina ratio of a thickness of a layer of the first material to a thicknessof a layer of the second material is from 1:1 to 3:2.
 4. The method ofclaim 1, wherein the reflective film stack comprises from 35 to 45 pairsof layers of the first material and the second material.
 5. The methodof claim 1, wherein the patterned energy source comprises extremeultraviolet (EUV) radiation.
 6. The method of claim 5, wherein a dose ofthe patterned energy source is less than 30 mJ/cm².
 7. The method ofclaim 5, wherein the photosensitive layer is exposed to the patternedenergy source for less than 100 milliseconds.
 8. A method comprising:depositing a planarity layer over a target layer, the target layerhaving a non-planar top surface; planarizing the planarity layer using achemical mechanical planarization (CMP) process; forming a multi-layerphotoresist over the planarity layer and the target layer, whereinforming the multi-layer photoresist comprises: forming a reflective filmstack comprising alternating layers of a first reflective material and asecond reflective material, wherein a ratio of a refractive index of thefirst reflective material to a refractive index of the second reflectivematerial is from 1.05 to 1.10; and forming a photosensitive layer overthe alternating layers of the reflective film stack; patterning thephotosensitive layer and the reflective film stack; and patterning thetarget layer using the reflective film stack as a mask.
 9. The method ofclaim 8, wherein the second reflective material comprises molybdenum(Mo).
 10. The method of claim 9, wherein the first reflective materialcomprises silicon (Si).
 11. The method of claim 9, wherein the firstreflective material comprises beryllium (Be).
 12. The method of claim 8,wherein a ratio of thicknesses of the layers of the first reflectivematerial to thicknesses of the layers of the second reflective materialis from 1:1 to 3:2.
 13. The method of claim 8, wherein patterning thephotosensitive layer comprises exposing the photosensitive layer to aradiation beam comprising patterned extreme ultraviolet radiation. 14.The method of claim 13, wherein the reflective film stack comprises from35 to 45 pairs of the first reflective material and the secondreflective material.
 15. The method of claim 14, wherein the radiationbeam is reflected at an interface between the first reflective materialand the second reflective material.
 16. A method comprising: depositinga target layer over a semiconductor substrate, wherein the target layeris deposited with a non-planar top surface; planarizing the targetlayer; after planarizing the target layer, forming a reflective filmstack comprising alternating layers of a first material and a secondmaterial different from the first material over the target layer;exposing a front side of a photosensitive layer to a radiation beam, theradiation beam comprising extreme ultraviolet (EUV) radiation, wherein aportion of the radiation beam is reflected from an interface between thefirst material and the second material of the reflective film stack to abackside of the photosensitive layer; exposing the photosensitive layerto a developer to remove a portion of the photosensitive layer exposingthe reflective film stack; etching the reflective film stack using thephotosensitive layer as a mask; and etching the target layer using thereflective film stack as a mask, wherein the target layer comprises asemiconductor material, a conductive layer, or a dielectric layer. 17.The method of claim 16, further comprising: depositing a planarity layerover the target layer; and planarizing the planarity layer and thetarget layer using a chemical mechanical planarization (CMP) process,wherein the reflective film stack is formed over the planarity layer.18. The method of claim 16, further comprising exposing the reflectivefilm stack to an adhesion promoter, the adhesion promoter comprisinghexamethyldisilazane (HMDS).
 19. The method of claim 16, wherein thereflective film stack is formed directly on the target layer.
 20. Themethod of claim 16, further comprising forming an etch selectivity layerover the target layer, wherein the reflective film stack is formed overthe etch selectivity layer, wherein an etch selectivity between the etchselectivity layer and the target layer is greater than an etchselectivity between the reflective film stack and the target layer.